No. 18/July 25, 1997
Waiting to See What Happens with European Corn Borers
After a few first-generation corn borer sieges in some scattered spots in the state, most folks are merely waiting to see what will come next. If you are tracking accumulations of degree-days to predict development of European corn borers, you can refer to the table printed in issue no. 16 (July 11, 1997) of this Bulletin. However, the long-term appearance of larvae from June through July suggests that timing control measures, if needed, will be difficult at best.
As has occurred occasionally (more often than not) in the past few years, several folks have reported a "continuous" flight of moths. The consequences of prolonged flights of adults and oviposition periods by females are larvae in all stages of development in any given field. Some observers still are finding second and third instars feeding among the leaves and fourth and fifth instars tunneling inside stalks. In these circumstances, one wonders if the field should have been treated earlier, or if the grower would benefit from a treatment now. Tough questions, questions that usually are associated with the second generation of European corn borers. Why are these prolong moth flights occurring?
Some of you may recall our speculative explanation during previous years when this has happened. Entomologists have determined that there are three "ecotypes" of European corn borers throughout North America. These ecotypes are determined by generations completed per season: one generation per season (univoltine), two generations per season (bivoltine), and three or more generations per season (multivoltine). Throughout Illinois during most years, the bivoltine ecotype predominates, although the multivoltine ecotype occurs in southern counties. The univoltine ecotype generally is distributed through the northern halves of South Dakota, Minnesota, Wisconsin, and Michigan, as well as north into Canada.
Some evidence suggests that we can experience both the bivoltine and univoltine ecotypes during some years in Illinois. In essence, the single peak of moth flight of the univoltine ecotype fits snugly between the two peaks of moth flight of the bivoltine ecotype. Voila, a seemingly "continuous" moth flight, making this pest more difficult than ever to manage.
As future research is conducted, we'll try to keep you abreast of any verification of this phenomenon. If it becomes commonplace, as it has in some of the northern states where the two ecotypes mingle, management decisions will need to be altered.
Kevin Steffey, Extension Entomology, (217)333-6652